The invention relates to buckles for seat belts and, more particularly, to an improvement of a buckle for a seat belt having a double lock mechanism.
Conventional buckles designed for seat belts for keeping passengers and the like securely seated include a buckle base, a latch mechanism for releasably latching a tongue plate to an end of the buckle base, and a buckle cover for enclosing the latch mechanism. The buckle base is firmly secured to the body of an automobile through brackets, wires, or the like, and transmits a tensile load acting upon the webbing to the body of the automobile at the time a collision of automobiles is accidentally occurred. Further the latch mechanism allows the tongue plate to be detachably coupled with the buckle base to thereby facilitate the fastening and releasing of the seat belt. When the tongue plate is inserted into the buckle base, a latching member prevents the tongue plate from being extracted by engaging a latching hole of the tongue plate.
The seat belt buckle of this type includes a buckle having a so-called double lock mechanism. In the double lock mechanism, the locking member keeps the latching of the latching member positively locked so that the tongue plate can be kept latched even if the buckle is subjected to impact after the tongue plate has been fitted into the buckle base by insertion, and the locking member can be unlocked by a simple operation of an operating member so that the latching of the tongue plate can be released.
However, in the buckle having such the double lock mechanism, the locking member, which is biased by a spring in a locking direction to block the latching member from rotating in an unlatching direction and thereby keeping the tongue plate latched by the latching member, might move from the locking position if large impact such as a collision of automobiles is accidentally applied in the unlocking direction that is opposite to the biasing direction. Further, the tongue plate might be unlatched if an inertial force is applied in an operating member operating direction and the operating member has moved by such inertial direction.
To overcome these problems, the following buckle is disclosed in Japanese Patent Unexamined Publication No. Hei. 3-222904 (1991). The latching member is arranged so as to be swingable and horizontally movable in the same direction as the tongue plate inserting and extracting direction. When an inertial force is applied to the buckle in such a direction as to cause the locking member to move from the locking position, the buckle is designed such that the catch of the latching member is held by a holding member provided on the buckle cover or the buckle base so as to allow the latching member to keep latching the tongue plate, or such that an auxiliary locking member moves to the locking position so as to allow the latching member to keep latching the tongue plate before the locking member moves from the locking position by an inertial force.
Further, another buckle proposed in Japanese Patent Unexamined Publication No. Hei. 3-277301 (1991) includes a swingable movement control member on the operating member, so that when an inertial force acts on the buckle in such a direction as to cause the locking member to move from the locking position, the movement control member can block the locking member from moving and the latching member can thereby keep latching the tongue plate.
However, the designing of the components of the thus constructed seat belt buckles is difficult. In addition, the operation of the lock mechanism that keeps the latching member latching the tongue plate is liable to be unstable, and the operability thereof under the normal condition is easy to be impaired.
That is, in the case of the buckle in which the catch of the latching member is held by the holding part arranged on the buckle cover or the buckle base to keep the latching member latching the tongue plate, the latching member must be arranged so as to be swingable and horizontally movable. It is for this reason that the operation of the latching member is liable to be unstable.
Further, in the case of the buckle having the auxiliary locking member, if the operating member moves in the operating direction faster than the auxiliary locking member due to impact, the locking member is caused to move from the locking position before the auxiliary locking member moves to the locking position to keep the latching member latching the tongue plate. As a result, the latching member may, in some cases, fail to keep latching the tongue plate.
Still further, in the case of the buckle having the movement control member for blocking the movement of the locking member, the movement of the locking member due to impact cannot be blocked unless the locking member moves from the locking position before the operating member. This requires that initial resistances necessary for operating the respective components be set correctly. Such setting is difficult. In addition, if the latching member is located at the unlatching position under the normal condition, the movement control member is caught by the latching member. This keeps the operating member from returning to the original position with the operating member remaining pushed.